The present invention relates generally to floating platform systems adapted for the exploitation of hydrocarbon formations found in offshore waters. More particularly, the invention relates to mono-column tension leg platforms (TLP) for recovery of deep sea hydrocarbon reserves.
The exploration for oil and gas deposits in offshore waters, and recovery of the oil and gas therefrom, is very expensive. As the water depth increases, the cost of exploration and production increases dramatically. Large capital expenditures are required to develop deepwater fields and thus only large and prolific oil and gas deposits can be profitably developed. Smaller oil and gas deposits usually do not justify large capital investments and therefore are deemed to be uneconomical to produce. There continues to be a need, therefore, for improved platform and drilling systems, particularly for use in deep waters, which would justify the economic investment to produce relatively small oil and gas fields.
Drilling and production platforms, such as TLP platforms, have heretofore been used to drill and produce deep water hydrocarbon formations. A TLP typically comprises a floating platform anchored to foundation members embedded in the seabed. Tension legs or tendons secure the TLP to the foundation members. The tendons are maintained in tension at all times by maintaining the buoyancy of the TLP significantly above the maximum TLP payload requirements under all environmental conditions.
A subcategory of TLP platforms, known as a mono-column TLP, has been developed to reduce costs and permit the economic development of smaller deepwater deposits of oil and gas. Mono-column TLP platforms are characterized by a single surface-piercing buoyant column with three or more pontoons extending radially outward from the single buoyant column to increase tendon spacing and effectiveness.
The upper portion of the mono-column TLP extends above the water surface and is subjected to forces developed by the wind. The lower portion of the TLP extending below the water surface is subjected to forces exerted by waves and current. The tendons secure the mono-column TLP to the seabed and effectively eliminate heave, pitch and roll motions. The tendons therefore suffer variations in tendon forces which cause tendon fatigue, and in severe cases, could lead to failure of a tendon. Very large tendon force variations during severe storms can slack tendons and lead to snap loads that could also cause tendon failure. Consequently, tendon design is driven by these force variations. In deep water, large tendon force variations can dramatically increase tendon system cost by leading to increases in tendon diameter, wall thickness, material properties, and connector size.
It is therefore an object of the present invention to provide a floating platform system which reduces the magnitude of force variations in the tendon system and therefore reduces the overall cost of the floating platform system to be installed in very deep water, where tendon system cost is a large component of overall floating platform system cost.
The present invention provides a mono-column TLP system for recovery of oil and gas from offshore oil and gas fields. The platform supports one or more decks above the water surface to accommodate equipment for drilling and/or processing oil, gas and water recovered from the oil and gas field. In a preferred embodiment, the platform includes a central column having a portion substantially below the water surface, and including a portion which extends above the water surface. The central column includes a base and is anchored to the seabed by one or more tendons secured to the base of the central column and to the seabed. The floating platform includes a ballast system which adjusts tendon pretension and adjusts the vertical center of gravity and mass of the platform system.